Heretofore, with regard to a polyethylene or an ethylene-.alpha.-olefin copolymer, its primary structure has been controlled by adjusting molecular weight, a molecular weight distribution or copolymerization properties (random properties, a blocking tendency and a branching degree distribution), or by adding a third component such as a diene so as to introduce branches.
On the other hand, for ethylenic polymers, various molding methods are usable, and typical known examples of the molding methods include injection molding, extrusion, blow molding, inflation, compression molding and vacuum forming. In such molding methods, the impartment of high-speed molding properties and the reduction of molding energy have been investigated for a long period of time in order to improve working properties and to thus lower a working cost, and so it is an important theme that optimum physical properties suitable for each use is imparted and the molding can be carried out with the optimum working properties.
In recent years, it has been elucidated that a uniform metallocene catalyst is excellent in the copolymerization properties between olefins, can obtain a polymer having a narrow molecular weight distribution, and has a much higher catalytic activity as compared with a conventional vanadium catalyst. Therefore, it has been expected that the metallocene catalyst will be developed in various technical fields by the utilization of such characteristics. However, a polyolefin obtained by the use of the metallocene catalyst is poor in molding and working properties, and for this reason, the application of the metallocene catalyst to the blow molding and the inflation is unavoidably limited.
A conventional known low-density polyethylene (LDPE) can be obtained by the high-pressure radical polymerization of ethylene and has both of long-chain branches and short-chain branches. It has been considered that the long-chain branches can be formed by the intermolecular hydrogen transfer reaction between the radical growth terminal of the polymer and the polymer. On the other hand, with regard to the mechanism for forming the short-chain branches, some theories have been reported. For example, a back-biting mechanism has been suggested [J. Am. Chem. Soc., Vol. 75, p. 6110 (1953)]. This suggested theory rationally explains that the butyl branch can be formed by the transfer of hydrogen after the formation of a six-membered intermediate at the growth radical terminal. According to another theory, it has been reported that the butyl branch is formed by the production of an associate of two ethylene molecules under a high pressure and a hydrogen transfer reaction at the radical growth terminal, and an ethyl branch can be introduced owing to the production of 1-butene by the hydrogen transfer reaction in the associate of two ethylene molecules [Makromol. Chem. Vol. 181, p. 2811 (1981)]. According to still another theory, it has been reported that the formation of the ethyl branch is accomplished by the transfer of hydrogen from the main chain of the polymer to an ethyl branch radical [J. Polym. Sci., Vol. 34, p. 569 (1959)].
As understood from the foregoing, it can be summarized that the formation of the long-chain branches and the short-chain branches in the low-density polyethylene is carried out by (1) the hydrogen transfer reaction based on a radical polymerization and (2) the change of radical polymerization reactivity by the association of ethylene molecules under a high pressure, and this is a usually recognized reaction mechanism. Therefore, in the above-mentioned reaction process, it is impossible to optionally control the amounts of the existing long-chain branches and short-chain branches as well as the number of carbon atoms in the short-chain branches. In particular, there have been limited the introduction and control of a methyl branch, a propyl branch, a hexyl branch and a short-chain branch derived from a branched .alpha.-olefin (e.g., a 4-methylpentene-1 branch).
Such a low-density polyethylene has a large melt tension and the large activation energy of melt flow by virtue of the long-chain branches, and therefore it is excellent in high-speed molding properties and suitable for the formation of films. However, since having a wide molecular weight distribution and containing a low-molecular weight component, the low-density polyethylene is inconveniently poor in environmental stress crack resistance (ESCR) and impact resistance.
On the other hand, various ethylenic polymers have been disclosed in which the long-chain branches are introduced into a high-density polyethylene skeleton. For example, there have been disclosed (1) an olefin copolymer having the long-chain branches obtained by the use of an .alpha.,.omega.-diene or a cyclic endomethylenic diene (Japanese Patent Application Laid-open No. 34981/1972), (2) a process for preparing a copolymer containing a higher non-conjugated diene content in a high-molecular weight segment than in a low-molecular weight segment which comprises carrying out polymerization in two steps to copolymerize the non-conjugated diene with an olefin (Japanese Patent Application Laid-open No. 56412/1984), (3) an ethylene-.alpha.-olefin-1,5-hexadiene copolymer obtained by the use of a metallocene/aluminoxane catalyst (Japanese Patent Application PCT-through Laid-open No. 501555/1989), (4) a process for introducing the long-chain branches by copolymerizing an .alpha.,.omega.-diene and ethylene in the presence of a catalyst comprising a zero-valent or a divalent nickel compound and a specific aminobis(imino) compound (Japanese Patent Application Laid-open No. 261809/1990), and (5) a polyethylene containing both of the short-chain branches and the long-chain branches which can be obtained by polymerizing ethylene alone by the use of the same catalytic component as in the above-mentioned (4) (Japanese Patent Application Laid-open No. 277610/1991).
However, in the copolymer of the above-mentioned (1), a crosslinking reaction takes place simultaneously with the formation of the long-chain branches by the diene component, and at the time of the formation of a film, a gel is generated. In addition, melt properties inversely deteriorate, and a control range is extremely narrow. Moreover, there is a problem that copolymerization reactivity is low, so that low-molecular weight polymers are produced, which leads to the deterioration of physical properties inconveniently. In the preparation process of the copolymer described in the aforesaid (2), the long-chain branches are introduced into the high-molecular weight component, so that the molecular weight noticeably increases due to crosslinking, and thus insolubilization, nonfusion or gelation might inconveniently occur. Furthermore, the control range is narrow, and the copolymerization reactivity is also low, and hence, there is a problem that owing to the production of the low-molecular weight polymers, the physical properties deteriorate inconveniently. In the copolymer of the above-mentioned (3), a molecular weight distribution is narrow, and for this reason, the copolymer is unsuitable for blow molding and film formation. In addition, since branch points are formed by the progress of the cyclizing reaction of 1,5-hexadiene, an effective monomer concentration is inconveniently low. In the process for introducing the long-chain branches described in the above-mentioned (4), there is a problem that a range for controlling the generation of a gel and the physical properties is limited. In addition, the polyethylene of the above-mentioned (5) is a polymer which contains neither ethyl branches nor butyl branches, and therefore the control of the physical properties, for example, the control of density is accomplished by methyl branches, so that the physical properties of the polyethylene tend to deteriorate.
Furthermore, there has been disclosed a method for preparing an ethylenic polymer to which working properties are imparted by the utilization of copolymerization, for example, a method which comprises forming a polymer ([.eta.]=10-20 dl/g) by preliminary polymerization, and then preparing an ethylene-.alpha.-olefin copolymer by main polymerization (Japanese Patent Application Laid-open No. 55410/1992). This method has an effect that melt tension can be increased by changing the melt properties of the obtained copolymer, but it has a drawback that a film gel tends to occur.
In addition, there have been disclosed ethylenic polymers obtained in the presence of a metallocene catalyst and methods for preparing the same, for example, (1) a method for preparing an ethylenic polymer in the presence of a constrained geometrical catalyst and an ethylenic copolymer obtained by this method (Japanese Patent Application Laid-open No. 163088/1991 and WO93/08221), (2) a method for preparing a polyolefin in the presence of a metallocene catalyst containing a porous inorganic oxide (an aluminum compound) as a carrier (Japanese Patent Application Laid-open No. 100808/1992), and (3) an ethylene-.alpha.-olefin copolymer which can be derived from ethylene and the .alpha.-olefin in the presence of a specific hafnium catalyst and which has a narrow molecular weight distribution and improved melt flow properties (Japanese Patent Application Laid-open No. 276807/1990).
However, in the technique of the above-mentioned (1), the control of density and the like can be accomplished by introducing an .alpha.-olefin unit into an ethylene chain, and the resultant product is a substantially linear polymer. According to the preparation method of the above-mentioned (2), the obtained copolymer of ethylene and the .alpha.-olefin has a large die swell ratio, but in view of the relation of the die swell ratio to the melting point of the ethylene-1-butene copolymer, it is apparent that the die swell ratio deteriorates with the rise of the melting point. Therefore, any copolymer cannot be provided in which the die swell ratio regarding a neck-in which is a trouble at the time of the formation of a film or a sheet is controllable in a wide melting point range.
On the other hand, the copolymer disclosed in the above-mentioned (3) contains an .alpha.-olefin unit as an essential unit, and it does not cover a copolymer having a resin density of more than 0.92 g/cm.sup.3. Furthermore, in the above-mentioned (1) and (3), when the branches are introduced, the melting point and the mechanical strength of the ethylene-.alpha.-olefin copolymer noticeably deteriorate.